Martensitic low alloy plate steel



United States Patent 3,258,372 MARTENSITHC LOW ALLOY PLATE STEEL Oscar 0. Miller, Westfield, and John L. Hurley, Bloomfield, N.J., assignors to The International Nickel Company, Inc, New York, N.Y., a corporation of Delaware No Drawing. Filed Jan. 21, 1963, Ser. No. 252,626

6 Claims. (Cl. 148-36) The present invention relates to alloy steels and, more particularly, to martensitic low alloy steels of relatively low cost which in the quenched and tempered condition manifest a combination of metallurgical properties of such magnitude that the steels are especially suitable as plate steels in section sizes up to at least 1 inch in thickness for general industrial and commercial use.

As is known, quenched and tempered plate steel is used in a substantial number of diversified industrial and commercial activities, including transportation, machinery, mining, power, petroleum, etc., and, thus, represents a significant tonnage item of the steel industry. More recently, it would appear that the rapidly developing field of cryogenics holds considerable potential for plate steels which are of low cost and which manifest good impact properties at low temperatures. Because of the versatile role characteristic of plate steels, it is not unexpected to find that they must possess a combination of properties capable of meeting the varying requirements imposed by the increasingly stringent demands of industrial and commercial use. In this regard, a satisfactory plate steel, of say 1 inch in thickness, should, in the quenched and tempered (at 1150 F. to 1200 F.) condition, exhibit a yield strength of at least 90,000 p.s.i., a Charpy V-notch impact value of at least 40 ft.-lbs. at room temperature and at least 15 ft.-lbs. at temperatures of 150 F. and lower, good ductility and toughness to insure ease of formability, and good weldability without compromising the other necessary properties. In addition and of significant importance from the commercial viewpoint, such properties must be achieved without incurring processing difiiculty, must be reproducible from heat to heat and the steels must be economical to produce.

Presently available plate steels, if they manifest the foregoing combination of properties, are of comparatively high cost and/or are attended by processing difliculties such as marked embrittlement problems upon slow cooling from tempering temperature. Some plate steels suffer from the inability of giving reproducible results, i.e., the properties of the steels vary from heat to heat to an undesirable degree. Other plate steels in the quenched and tempered condition do not afford the aforementioned combination of properties; for example, some steels are prone to crack upon welding while others exhibit a toughness which is marginal at best or unsatisfactory at low temperatures.

It has now been discovered that martensitic low alloy steels containing copresent special amounts of carbon, manganese, silicon, nickel, chromium, molybdenum, a'luminum and boron manifest the aforedescribed combination of properties when quenched and thereafter tempered at temperatures above 1100 F. In addition, the steels are of relatively low cost, can be processed without difficulty and the properties characteristic thereof are reproducible from heat to heat. These additional factors render the steels particularly attractive for use as steel plates.

In accordance with the present invention, an optimum free combination of the properties referred to above herein is achieved with alloy steels having the following most advantageous composition: about 0.15% to about 0.22% carbon, about 0.65% to about 1% manganese, about 0.2% to about 0.35% silicon, about 0.7% to about 1% nickel, about 0.2% to about 0.35% chromium, about 0.15% to about 0.3% molybdenum, about 0.02% to about 0.06% aluminum, about 0.0005 to about 0.004% boron, and the balance essentially iron.

To continuous-1y achieve optimum results it is most advantageous that the alloy ranges set forth above be maintained. To obtain the necessary strength (hardness) and hardenability levels the carbon content should not be appreciably lower than 0.15 With significantly lower carbon contents, e.g., 0.1%, free ferrite can form readily enough to give a marked reduction in hardenabi'lity. Moreover, processing difficulty can be encountered with carbon levels of say 0.1%, as opposed to 0.15%. With 0.15% carbon a much cleaner steel is provided because less oxygen is needed in producing it than would be required in producing the steel if it were to contain 0.1% carbon. Less oxygen will be present in the molten steel bath at equilibrium for any given temperature at the 0.15 carbon level than for 0.1% carbon; thus, there is a greatly reduced tendency to form oxides in the steel and less oxygen will remain in the steel. The carbon content should not fall below 0.14% and for optimum weldability it is desirable not to exceed 0.22% carbon.

With regard to the silicon content, it is advantageous that the amount of silicon not exceed 0.35%. Silicon should be added prior to the aluminum for good preliminary deoxidation, accompanied by fiuxing and removing silica to the slag. This insures minimizing retention of dissolved oxygen and keeping it to a low level, thereby forming only small amounts of alumina. High silicon contents adversely affect certain properties and give rise to dirty steels which are known to be difficultly processable. The siilcon content should always be less than 0.5%.

Manganese, nickel, chromium and molybdenum contribute to afiording high tensile strengths (hardness), high yield strengths and high hardenability. In addition, nickel greatly contributes to the toughness of the steels, especially to good impact properties at low temperatures. Each of these elements retard softening upon tempering although molybdenum and chromium are most effective in this regard. However, amounts of these four elements significantly above the aforedescribed ranges will undesirably lower the M temperature of the steels and tend to cause cracking upon quenching from austenitizing temperature. Further, since the steels within the invention are readily adapted to be used under the severe conditions imposed by submerged arc welding, cracking can occur if the amounts of manganese, nickel, molybdenum and chromium are too high. In addition, appreciably higher amounts of either or both manganese and chromium promote or contribute to bringing about a serious problem of temper embrittlement. This is obviously disadvantageous for steels especially adapted {for use in the quenched and tempered condition as are the steels of the present invention. It is to be also pointed out that high amounts of chromium, e.g., 0.55%, tend to decrease toughness in the alloy steels contemplated herein, i.e., alloy steels having a tempered martensitic microstructure. On the other hand, low chromium contents, e.g., 0.1%, can be quite detrimental since the ability of the steels much greater alloy content would be necessary to achieve the desired hardenability level and, as a consequence, added cost would result. However, a boron content appreciably above that contemplated herein creates an embrittling effect.

Alloy steels within the foregoing ranges have more than an adequate degree of hardenability since they harden to at least 90% martensite upon quenching sections up to at least 1 inch thick from austenitizing temperatures. This is important in order to afford good impact properties and a high yield strength to tensile strength ratio, e.g., 0.8

or higher. Further, a margin of safety is required to insure complete hardening under commercial conditions of operation. When austenitized, quenched (cooled rapidly enough to give martensite), and tempered at temperatures at or above 1100 F., e.g., 1150 F., the steels are characterized by a yield strength (0.2% offset) of at least 90,000 p.s.i., good toughness as shown by a Charpy V-notch impact strength of at least 40 and as high as 90 ft.-lbs. at room temperature and, more importantly, of

at least 15 ft.-lbs. at temperatures at least as low as 150 F., good ductility as evidenced by a tensile elongation in 2 inches of at least 20% on a standard 0.505-inch diameter tensile specimen together with reductions of .area of at least 50%, and ease of fabrication combined with good weldability. Thus, the invention provides a low alloy steel which when quenched and tempered develops a high yield strength usually in excess of 95,000 or 100,000 p.s.i., with an impact transition temperature below -150 F. in plate form and in weldments. The foregoing properties obtain in sections of at least 1 inch thickness, are reproducible from heat to heat and, thus, the steels are especially adaptable for use as plate steels.

The presence of vanadium, titanium and other strong carbide formers is to be avoided. To dissolve such carbides requires the use of extremely high austenitizing temperatures, e.g., 1800 F. to 2200 F. This promotes the formation of a large grain size which, as is known, detrimentally affects impact strength (toughness). Additionally, titanium, for example, particularly in the presence fore, the effect of titanium and vanadium upon hardenability will be an undesirable variable. There are at least two opposing factors involved. When these elements are in solution in austenite, they strongly increase harden ability. However, when these elements are not in complete solution in austenite, they act to strongly decrease hardenability. Therefore, unless very high austenitizing temperatures are employed, there is doubt under normal processing conditions as to how much titanium and/or vanadium is in solution. This affects not only hardenability, but also affects tensile strength upon tempering. That is to say, during tempering a significant variation in tensile strength can be noted. Extremely small amounts of such elements up to 0.01% of each can be tolerated; however, the total amount of such elements should not exceed 0.02%.

Satisfactory results can also be obtained with steels of the following composition: about 0.14% to about 0.24% carbon, about 0.6% to about 1.2% manganese, about 0.10% to less than about 0.5% silicon, about 0.6% to about 1.5% nickel, from 0.18% to less than 0.4% chromium, about 0.15% to about 0.35% molybdenum, about 0.015% to about 0.1% aluminum, about 0.0005% to about 0.005% boron, and the balance essentially iron.

In carrying the invention into practice, the steels should be austenitized at about 1625 F. to about 1750 F. and preferably between 1650" F. and 1700 F. Holding at such temperature for about 20 minutes or more is quite adequate. Quenching should be conducted sufficiently rapid to insure a structure of at least 90% martensite. Water quenching is quite adequate and is desirable in quenching sections /z-inch thick and above for optimum results. Tempering at a temperature of 1000 F. to 1200 F. provides a steel which does not manifest an appreciable decrease in impact properties at ambient temperatures. All factors considered, tempering at 1100 F. to 1175 F., e.g., 1150 F., is quite satisfactory, although actual tempering temperature will be dictated by commercial application. The steels should be held at tempering temperature for a period consistent with good commercial heat treating practice. Periods of '1 to 2 hours have been found quite adequate although shorter periods, e.g., onehalf hour, can be used satisfactorily.

For the purpose of giving those skilled in the art a better understanding of the invention and/or a better ap preciation of the advantages of the invention, the following illustrative data are given.

A series of alloys was prepared having compositions of boron, tends to or is capable of reducing the impact as given in Table I.

TABLE I Alloy Percent Percent Percent Percent Percent Percent Percent Percent Percent 0 Mn Si Ni Cr Mo Al B Fe A 0. 16 0. 67 19 0. 74 0. 23 0. 17 0. 03 0. 003 Bal. B 0. 18 0.83 0. 23 0.86 0. 26 0. 26 0. 035 0. 004 Bal. C 0. 21 0. 91 0. 33 0. 96 0. 31 0. 26 0. 037 0. 004 Bal.

1 Determined as acid soluble aluminum.

toughness of the steel when held at tempering temperatures and presents an embrittlement problem upon cooling from tempering temperature. As referred to before herein, a special characteristic of the alloy steels of the instant invention is found in the fact that the mechanical properties thereof are reproducible from heat to heat, i.e., the properties do not change to any significant degree. However, if vanadium and/ or titanium are present to any appreciable extent, difiiculty could arise with regard to ob- The steels of Table I were prepared in an air induction furnace, the boron being added just before pouring. The steels were forged to plate l-inch thick and 4 /2 inches wide. The plates were austenitized at 1675 F. for 1 hour, water quenched, tempered for 2 hours at 1150 F. (Alloys A, B and C) or 1200 F. (Alloys AA, BB and CC, which alloys are the same as A, B and C, respectively) and then water quenched to be consistent with and to simulate commercial heat treating practice (but the properties are unaffected by slowly cooling, such as in air, from tempering temperature). Duplicate 0.505-inch diameter tensile specimens were machined from each plate. Charpy V-notch impact specimens were machined (using standard technique) so that they were parallel to the longitudinal axis of the plates and were notched perpendicular to the surface. In addition to determining impact values at room temperature, a specific low temperature test was employed, to wit, determination of the lowest temperature at which the steels exhibited an impact value of at least 15 ft.-lbs. The Charpy keyhole-notch test was avoided because this testing procedure is not deemed sufliciently severe and the higher results that would be expected would not be considered as snfficiently discriminating indicators of impact resistance. The results of the tests are given in Table II.

dinal and transverse directions and the results are given in Table III.

TABLE III 1 Yield Strength Tensile Strength, Elongation in (0.2% oilset), p.s.i. percent p.s.i.

Long 104, 600 114, 200 18. 6 Trans 106, 300 113, 800 17. 2

1 Each value represents the average of 4 specimens.

1 Elongation, using a gage length of 2 inches.

The data in Table II illustrates that yield strengths (0.2% offset) of greater than 100,000 p.s.i. are obtained in steels within the invention after tempering at 1150 F. The Charpy V-notch impact strength of these steels is also excellent, particularly at low temperatures. In all the tests, the impact transition temperature with a ft.-lb. criterion was well below 150 F. In addition, the ratio of yield to tensile strength for each alloy was greater than 0.85. Accordingly, less alloy material is required than might otherwise be the case with competitive steels having lower vield to tensile strength ratios.

A l-inch thick plate of an alloy steel within the invention produced in a commercial steel mill and having a composition about as follows: 0.17% carbon, 0.71% manganese, 0.23% silicon, 0.8% nickel, 0.33% chromium, 0.2% molybdenum, 0.002% boron and 0.022% aluminum (acid soluble aluminum), was tested to ascertain the properties obtained under commercial operation. This steel was austenitized at between 1625 F. and 1675 F. and the tempering temperature was maintained at between 114- 0" F. and 1160 F. This commercial heat had the following properties: a yield strength of 103,000 p.s.i., a tensile strength of 112,000 p.s.i., a tensile elongation (2- inch gage length on 0.505-inch bar) of 23%, a reduction Table III illustrates the relatively small dilference in tensile ductility. Thus, severe forming operations can be performed in either direction with assurance. It should be pointed out that a 2-inch gage length was used in measuring ductility. If the conventional or standard gage length of 1.4 inches were used (since the specimens were 0.3 inch thick), the elongation values would be over 20%.

Weld-ability is a most important commercial characteristic of plate steels. If a plate steel is prone to or susceptible to cracking, or if, for example, strength properties are appreciably reduced as a result of welding, the effectiveness of the plate steel and its commercial acceptability are obviously impaired. Moreover, many steels require a stress-relief treatment subsequent to the. welding operation. It stress-relieving of the heat-affected zone can be avoided, the overall cost of the final product is considerably reduced. Further, stress-relieving large articles of manufacture is a most tedious and difficult task. To illustrate the good weldability characteristics and the effect of welding upon alloys within the instant invention, alloys were produced having compositions given in Table IV. Alloy D is an alloy within the invention whereas Alloy N 0. 1 responds to a prior art nickel-free alloy steel which is outside the present invention.

TABLE IV Chemical composition Alloy Percent Percent Percent Percent Percent Percent Percent Percent 0 Mn Si Ni Cr Mo A1 1 B 1 Determined as acid soluble aluminum.

2 Contained 0.04% zirconium.

of area of and a Charpy V-notch strength of 25 ft. lbs. at a temperature of 200' F. These data are indicative that alloy steels within the invention have an excellent combination of properties from the commercial aspect.

With respect to tensile ductility, a relatively small difference between longitudinal and transverse tensile ductility values has been observed. It is not unusual in prior art steels to find a substantial variation in longitudinal and transverse tensile ductility and in this regard transverse ductility is normally lower since the break occurs along the longitudinal axis. Employing the above-described commercial heat 0.3 inch plate specimens having a /2-inch width were prepared and tested in the longitu- The alloys were prepared by melting in an air induction furnace and were thereafter forged to plates having a thickness of 1 inch and a width of between 4 /2 to 5 inches. The plates were cut into lengths of about 13 inches and were given the following heat treatment: austenitized at 1675 F., water quenched, tempered for two hours at 1200 F. and thereafter water quenched. The plates were machined (including top and bottom surfaces) to 4 /2 inches wide and a groove was machined along the vertical face of one of the 13-inch edges of each plate which left a lip of about -inch. The grooves had a radius of about fii-inch and flared out at an angle of about 15 to the 11016 electrode. The Welding current was 190 amperes and the welding voltage was 25 volts. Using a speed of 6 inches per minute, the bead sequence consisted of a series of about passes to fill the groove, the final pass being made down the middle of the weld. The welded plate was then turned over and a single root bead was deposited on the reverse side. Subsequent to the welding operation the plates were cut in half, one-half being tested as-welded while the other half was tested after being stressrelieved at 1100 F. for two hours. No pre-heating was used prior to the Welding operation.

Tensile tests and Charpy V-notch impact tests were made on the base plate. Additonal tensile and Charpy V-n-otch tests were conducted to ascertain the properties of the heat-afi'ected zone in both the as-welded and stressrelieved conditions. In this connection, duplicate 0.505- inch diameter tensile specimens were cut parallel to the longitudinal direction of the base plate. Tensile specimens from the as-welded and the welded plus stressrelieved plates were cut from the transverse direction and were machined. In measuring the elongation, a 2-inch gage length was employed on the base plate and a l-inch gage length was used on the welded plate. Charpy V- notch specimens were cut from the transverse direction of the plate and were notched perpendicular to the surface. The specimens were machined to about 0.394- inch square blanks and macroetched to reveal the heataffected zone. The entire heat-affected zone as Well as a small amount of the weld metal and base metal were under the Charpy V-notch.

The tensile and the impact data determined by the foregoing tests are given in Tables V and VI, respectively.

TABLE V Tensile properties 1 not harmful) which is not the case with many prior art steels. Thus, neither a pro-conditioning heat treatment nor a subsequent stress-relief treatment is required.

Since alloy steels within the invention are especially adapted for gas and liquid pipe lines, explosion bulge tests were conducted to further confirm that the steels in both the non-welded plate and welded plate conditions possess levels of toughness and resistance to brittle fracture of a magnitude sufficient to withstand the requirements of commercial use. The test specimens, which had the composition given for the commercial heat set forth hereinbefore and treated as there described, were 14-inch squares, having a thickness of 0.281 inch and were of three types: (1) un-welded plate, (2) Welded plate in which the weld was deposited at a speed of 50 inches per minute, and (3) welded plate in which the weld was deposited at a speed of 75 inches per minute. Submerged arc welding technique was employed. Since under commercial conditions of operation, gas pressures of about 900 or 1000 p.s.i. are usual and temperature conditions seldom ever reach as low as -30 F. or F., (0 F. being a relatively low temperature), the tests were designed to include conditions of greater severity in order to insure the occurrence of fracture for purposes of evaluation. Pentolite was used as the explosive in 2 and 4 lb. charges and the explosive was varied in distance with respect to the specimens. A 2 lb. charge at distances of 15 and 18 inches develops a pressure of approximately 9000 p.s.i. and 6300 p.s.i., respectively. The test temperature was -80 F. Specimens of the un-welded plate did not fracture at 80 F. with explosive charges of 2 lbs. at 15 inches and 4 lbs. at

1 Failure occurred in base plate and not in weld metal or heat-affected zone.

2 1=Base Plate.

2=Heat-aficcted zone (as-welded condition). 3=Hcat-afiected zone (stressrelieved condition).

TABLE VI Impact transition temperature 1 Heat-Afiected Zone 1 15 ft.-lb. criterion.

The data in Tables V and VI confirm that welding does not adversely affect yield or tensile strength, impact strength or the ductility of the alloy steels within the invention. In fact, the properties are comparable to those set forth in Table II. In addition, inspection showed the welds to be crack free. Furthermore, welds can be deposited with the heat-affected zone showing good toughness to temperatures below 150 F. The data further illustrate that the overall combination of properties of Alloy D within the invention is much superior to that of Alloy No. 1. In this regard, the impact properties of Alloy D were markedly better in both the base plate and heat-affected zone. The data in Table V reflects that a stress-relieving treatment is not required (but is 24 inches. As will be noted from what has been stated above, such conditions are extremely severe and are markedly greater than those encountered in service. A 4 lb. charge at 15 inches resulted in blowing out the bulge area. The fracture was completely ductile indicating merely that the tensile strength of the specimen had been exceeded. With regard to the welded specimens, the buldge area was not blown out (in either the 50 i.p.m. or 75 i.p.m. weldments) using a 2 1b. charge at a distance of 18 inches. Relatively slight damage occurred notwithstanding the severity of the test conditions. One of the 50 i.p.m. weldments developed a hair-line crack along the edge of the weld bead and another showed cracking transversely acrossthe weld. In the 75 i.p.m. Weld condition fracture, as would be expected, was more extensive. It required a 2 lb. charge at 15 inches to blow out the bulge area in the welded specimens and in each instance one-half of the area Was blown out. These data illustrate that under severe test conditions the alloy steels within the invention manifest a high degree of toughness and are resistant to fracture.

To illustrate that the steels of the invention could be readily fabricated, an alloy steel similar in composition to Alloys A, B and C of Table I and containing 0.18% carbon, 0.8% manganese, 0.15% silicon, 0.84% nickel, 0.36% chromium, 0.27% molybdenum, 0.029% aluminum, 0.003% boron (added), and the balance essentially iron, was austenitized at 1700 F. for 1 hour, water quenched, tempered for 2 hours at 1200 F. and then water quenched. The steel was machined to a /2-inch thick specimen having a width of 1 inch and a length of 7 inches. The specimen was then subjected to a severe bend test which comprised bending the specimen over a /z-inch radius. Upon examination of the 180 bend, no cracks were observed and the test results confirmed the excellent formability characteristics.

A further feature of the invention is that under normal conditions of tempering or stress-relieving when alloy steels within the invention are slowly cooled from tempering temperature, no problem of temper embrittlement is incurred as is the case with some other alloy steels. Thus, if desired, alloy steels of the invention can be slowly cooled from tempering temperature, e.g., air cooled to minimize distortion and then put into service.

In view of the excellent combination of mechanical properties characteristic of the martensitic low alloy steels within the invention, the steels are suitable for wide commercial application, including pressure vessels, fluid storage containers, heavy machinery, railroad cars, structural members, e.g., frames for trucks, ship framework, boom-s, materials requiring exceptional resistance to impact, e.g., penstocks, etc. As indicated above, the steels are especially useful as steel plate, particularly in the fabrication of oil, gas and other fluid pipe lines therefrom.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A martensitic low alloy steel particularly adapted for use as steel plate in section sizes up to at least 1 inch in thickness and characterized when quenched and tempered at temperatures of at least 1100 F. by a yield strength (0.2% offset) of over 90,000 p.s.i., a tensile strength of over 100,000 p.s.i., a yield to tensile strength ratio of at least 0.8, a Charpy V-notch impact value of at least 40 -ft.-lbs. at room temperature and 15 ft.-lbs. at

temperatures of at least as low as l F., and good ductility and weldability, said alloy steel consisting essentially of about 0.15% to about 0.22% carbon, about 0.65% to about 1% manganese, about 0.2% to about 0.35% silicon, about 0.7% to about 1% nickel, about 0.2% to about 0.35% chromium, about 0.15% to about 0.3% molybdenum, about 0.02% to about 0.06% aluminum, about 0.0005% to about 0.004% boron, and the balance essentially iron.

2. A steel plate produced from the martensitic low alloy steel set forth in claim 1.

3. As a new article of manufacture, pipe for the transmission of gases and fluids and produced from the martensitic low alloy steel set forth in claim 1.

4. A low alloy steel adapted for use as steel plate and characterized by a martensitic structure in the quenched and tempered condition and by a good combination of mechanical properties, said alloy steel consisting essentially of from .14% to 0.24% carbon, 0.6% to 1.2% manganese, 0.1% to less than 0.5% silicon, 0.6% to 1.5% nickel, from 0.18% to less than 0.4% chromium, 0.15% to 0.35% molybdenum, 0.015% to 0.1% aluminum, 0.0005% to 0.005% boron, and the balance essentially iron.

5. A steel plate produced from the martensitic low alloy steel set forth in claim 4.

6. As a new article of manufacture, pipe for the transmission of gases and fluids and produced from the martensitic low alloy steel set forth in claim 4.

References Cited by the Examiner UNITED STATES PATENTS 2,004,138 6/1935 Story et al. 29180 X 2,354,147 7/1944 Scott 128.85 X 2,513,395 8/1947 Bardgett 75123 2,586,042 2/1952 Hodge et al. 75l28.4 2,798,805 7/1957 Hodge et al. 75123 3,110,586 11/1963 Gulya et al. 75l28.85 3,110,635 11/1963 Gulya et al. 75123 X 3,110,798 11/1963 Keay 75--124 X 3,178,279 4/1965 Nakamura 75l24 DAVID L. RECK, Primary Examiner.

H. TARRING, P. WEINSTEIN, Assistant Examiners. 

1. A MARTENSITIC LOW ALLOY STEEL PARTICULARLY ADAPTED FOR USE AS A STEEL PLATE IN SECTION SIZES UP TO AT LEAST 1 INCH IN THICKNESS AND CHARACTERIZED WHEN QHENCHED AND TEMPERED AT TEMPERATURES OF AT LEAST 1100*F. BY A YIELD STRENGTH (0.2% OFFSET) OF OVER 90,000 P.S.I., A TENSILE STRENGTH OVER 100,000 P.S.I., A YEILD TO TENSILE STRENGTH RATIO OF AT LEAST 0.8, A CHARPY V-NOTCH IMPACT VALUE OF AT LEAST 40FT.LBS. AT ROOM TEMPERATURE AND 15FT.-LBS. AT TEMPERATURES OF AT LEAST AS LOW AS -150F., AND GOOD DUCTILITY AND WELDABILITY, SAID ALLOY STEEL CONSISTING ESSENTIALLY OF ABOUT 0.15% TO ABUT 0.22% CARBON, ABOUT 0.65% TO ABOUT 1% MANGANESE, ABOUT 0.2% TO ABOUT 0.35% SILICON, ABOUT 0.7% TO ABOUT 1% NICKEL, ABOUT 0.2% TO ABOUT 0.35% CHROMIUM, ABOUT 0.15% TO ABOUT 0.3% MOLYBDENUM, ABOUT 0.02% TO ABOUT 0.06% ALUMINUM, ABOUT 0.0005% TO ABOUT 0.004% BORON, AND THE BALANCE ESSENTIALLY IRON. 